Flowing water is one of the oldest sources of power for a mechanical device. With the early development and increased use of electricity, water power was an obvious choice for a power source and large numbers of relatively small hydro-electric generators were widely used for many years. As cheap fuels became available, and electric power distribution systems increased in size, fewer small size hydro-electric generators were employed, and many of these were abandoned. However, as alternate power sources, and especially oil and gas, became increasingly expensive, and with the advent of government regulations requiring large power generating utilities to purchase excess electric power of small generating systems, it becomes ever more practical and economically feasible to utilize the large quantities of presently wasted water power for generation of electricity by relatively small scale systems.
Small hydro-electric generating systems are now feasible and economically desirable for a great many applications. A penstock supplying water from one reservoir to another can have a system installed. Various long-distance transport lines frequently involve water flow suitable for a hydro-electric generation. In large scale water distribution systems, as where a single water district collects water from a number of sources and distributes the water to many different cities or locations in a metropolitan area, it is common to employ a pressure reducing station to drop the relatively high pressure from the water district supply lines to a lower pressure at individual user cities or areas. A typical pressure reducing station simply creates a drop in the water pressure, entirely wasting the energy of the pressure difference. Such pressure reducing stations accordingly are prime candidates for application of small scale hydro-electric power generation, particularly since the surplus electricity of a small scale generator can readily be sold to a local electric power company.
In situations where the water employed for electricity generation is fed to a user system, the flow rate will vary widely on a daily basis as use by individual industries and households varies. Turbine efficiency varies with flow rate and in many situations the daily variation of flow rate of the user system may be too great to enable a single turbine generator to operate near maximum efficiency while making maximum use of the energy of the water flow.
Other problems exist in small hydro-electric systems. Where the generator is connected to a power grid, a failure in such grid may decrease the load on the generator to the point where turbine speed increases excessively. Excessive turbine speed significantly increases the flow resistance whereby flow rate through the turbine will be decreased. Accordingly, upon such power failure, the turbine generator system must be shut down. After initiation of shut down, the turbine speed continues at an excessive rate for a short period of time, until the control valve closes. Thus, it is not desirable to restart the system until the control valve has closed and the turbine speed has descreased significantly.
Where the generator of the turbine generator system is of the synchronous type, and the generated electricity is to be supplied to a power grid, the turbine generator must be brought up to a predetermined speed and the generator output must have a closely controlled phase and frequency, matching that of the power grid system, before it can be switched into the power system. Accordingly, controls for such a synchronous generator are quite complex. An induction type generator on the other hand does not require local phase and frequency control but requires a source of excitation.
Accordingly, it is an object of the present invention to provide a control for a turbine generator system that avoids or minimizes above-mentioned problems.